Golf balls are typically comprised of a cover that is injection molded or compression molded around a golf ball core and which may include one or more wound or solid layers and also a liquid or solid center. Also, the individual layers within the cover are generally either compression or injection molded.
Injection molding is generally conducted between two mold halves that together define a mold cavity in which the core is supported with fixed or retractable pins. In some processes, such as disclosed in U.S. Pat. No. 5,147,657, the support pins are retractable. Resinous cover material is injected at high pressures into the mold cavity, around the core. The retractable pins are withdrawn from the cover material when it is solid enough to support the core, yet soft enough to fill pin holes remaining where the pins once were as further material is injected.
Injection molding methods are conducted with the mold closed. Injection molding generally takes place with plastic pressures upwards of about 12,000 psi. These high pressures tend to deform the golf ball core by compressing portions thereof which are disposed adjacent the injection ports, causing portions of the core disposed away from the ports to extend. Also, injection molding equipment typically includes extremely small vents through which air contained within the mold cavity may exit as molding material is injected. The very limited venting speed achievable through these vents can limit the injection speed of the cover material or layer material.
Generally, the injected material is injected from more than one port around the core to speed the injection process. Where material flowing from the various ports meet, weld lines, or knit lines, can be formed, resulting in discontinuities and residual stresses across the weld lines. This produces poor finishes with poor definition of features that are molded into the layer or cover, such as dimples. Golf ball material failures also tend to occur at the weld lines after repeated use of the golf ball.
In addition, due to friction between the injected material and the mold cavity and core, the material catches at various locations on the mold halves and core during its injection, creating more internal stresses and further decreasing the homogeneity of the molded material. These stresses also tend to produce lower quality finishes and areas that are more prone to structural failure.
As shown in U.S. Pat. No. 4,501,715, covers have also been compression molded about golf ball cores. In this technique, previously molded hemispheres are placed around a core to form a shell. The assembly is then placed between two compression mold halves, which are then heated and pressed together. The shells are often thickest at their deepest point to enhance good surface formation and evacuation of entrapped gases.
In compression molding, neither pins or injection ports or gates are required, and details on the molded product, such as dimples, are generally visually significantly sharper than those resulting from injection molding. Compression molding, however, does necessitate the added step of molding the half shells before they are compression molded around the core. In addition, the finished half shells must be pre-aligned within the compression mold halves by hand or by machine, and a weak point results at the parting line where the material from the two shells has melted and flowed together.